The present invention relates to a photoconductive film type light receiving element for converting data on ordinary clerical documents into electrical signals in time-series and to a method of manufacturing the same.
More specifically, the invention relates to a film type light receiving element in which a photoconductive non-crystalline film is formed, for instance, by vacuum evaporation on a flat and smooth insulating substrate on which belt-shaped electrodes are arranged in a desired density and in which an optically transmissive electrode is formed on predetermined parts of the belt-shaped electrodes. A film of Au or the like is used for each of the belt-shaped electrodes, an Se-As-Te film is used for the photoconductive non-crystalline film, and a high and low specific resistance layer of the Sn oxide or a film of a target compound SnO.sub.2 or In.sub.2 O.sub.3 --SnO.sub.2 (hereinafter referred to as "ITO") which is obtained by sputtering under a gaseous pressure 3.times.10.sup.-3 -4.times.10.sup.-3 Torr of a mixture of argon and oxygen with the ITO as a target material are used for the optically transmissive electrode. Furthermore, the invention pertains to a method of manufacturing the film type light receiving element.
FIG. 1 is a sectional view showing the structure of a conventional film type light receiving element. Belt-shaped electrodes 2, a photoconductive film 3 and an optically transmissive electrode 4 are formed on a flat and smooth substrate 1 in the stated order.
For instance, the substrate 1 is made of a highly insulative material such as quartz glass or Vycor.TM. brand glass, the belt-shaped electrodes are formed of an Au film, the photoconductive film 3 is an Se-As-Te film or a CdSe film, and the optically transmissive electrode 4 is an In-Sn oxide film.
In FIG. 2, the curve I indicates specific resistance .rho. with oxygen partial pressure P.sub.o for the case where an SnO.sub.2 film is formed on a glass substrate by a magnetron type sputtering device using an SnO.sub.2 sintered substrate as a target. Both the specific resistances .rho. and the oxygen partial pressures P.sub.o are indicated by using logarithmic scales.
By forming on the flat and smooth substrate of Pyrex.TM. glass 1 the belt-shaped electrodes, i.e. the Au films 2, the photoconductive film, i.e. the Se-As-Te film 3, and the optically transmissive electrode, i.e. the SnO.sub.2 film 4 in the stated order, a light receiving element having a structure as shown in FIG. 1 is obtained.
In the conventional method, the electrical conductivity and the transmissivity of the optical transmissive electrode are considered important factors. However, the conventional method is disadvantageous in that the most important factor of the photoconductive conversion element, namely, the electric current ratio for light and dark thereof has been heretofore thought impossible to increase.
In order to eliminate the above-described difficulty, the inventors have proposed a method of manufacturing a film type light receiving element in which metal electrodes and a photoconductive film are provided in the form of layers on a flat and smooth substrate and an optically transmissive electrode film of tin oxide is formed thereon using a magnetron type sputtering technique wherein, by utilizing the fact that the electric current ratio for light and dark input intensities of the light receiving element varies according to the oxygen partial pressure of a sputtering atmosphere in the sputtering process and the ratio has a peak value, the oxygen partial pressure is set close to a value at which the electric current ratio for light and dark input intensities reaches its peak value. This method is described in U.S. patent application Ser. No. 242,736 filed on Mar. 11, 1981. This method is here specifically not described as nor is to be construed as being prior art so far as the instant application is concerned and is mentioned here only to aid in attaining a full understanding of the invention disclosed herein.
An Example of that method will be briefly described with reference to the case where the substrate 1 is an insulating glass or ceramic material having a high surface smoothness, the belt-shaped electrodes 2 are formed of an Au film with a Cr base, the photoconductive film 3 is an Se-As-Te series film, and the optically transmissive film 4 is an SnO.sub.2 film which is formed by a magnetron type sputtering device using an SnO.sub.2 sintered substance as a target.
The photoelectric conversion characteristic of the film type light receiving element having the afore-described structure depends on the quality of the material of each layer and the quality of each film. Especially the interface between the belt-shaped electrodes 2 and the photoconductive Se-As-Te film 3 and the junction interface between the Se-As-Te film 3 and the optically transmissive SnO.sub.2 film 4 greatly affect the electric current ratio for light and dark input intensities.
The Se-As-Te film 3 acts as a P type photoelectric element. Therefore, it is desirable that the light receiving element be operated with the belt-shaped electrode 2 as the negative electrode and with the optically transmissive SnO.sub.2 electrode 4 as the positive electrode. In this connection, it is preferable that the material of the negative electrode be Au, which has a low electron injection property, while the material of the positive electrode is an n type SnO.sub.2 film which has a low Hall injection property.
As described in the above-referenced patent application an important factor in manufacturing the SnO.sub.2 film is to use a magnetron type sputtering device with which the temperature of the substrate is increased only a little and the Se-As-Te film is not crystallized.
As is apparent from the curve I in FIG. 2, the point A at which the specific resistance .rho. is a minimum corresponds to a specific resistance of 2.times.10.sup.-1 .OMEGA.-cm and an oxygen partial pressure of 1.8.times.10.sup.-4 Torr. In this case, the gaseous pressure of a mixture of argon and oxygen is 3.0.times.10.sup.-3 Torr for a target of an SnO.sub.2 sintered body. Accordingly, if it is desired only to minimize the electrical conductivity, an optically transmissive electrode having an excellent electrical conductivity can be obtained by forming the electrode under the sputtering conditions defined by the point A.
The curve II in FIG. 2 indicates the relations between electric current ratios for light and dark intensities when a predetermined electric field is applied to the light receiving element and for a sputtering gas oxygen partial pressure P.sub.o. In FIG. 2, the electric current ratio for light and dark intensities becomes maximum at the point B. In this case, the oxygen partial pressure is 2.times.10.sup.-4 Torr and the electric current ratio for light and dark intensities is 400. As is clear from FIG. 2, the oxygen partial pressure at the point B is not always equal to that at the point A.
If the above-described light receiving element is operated in an accumulation type photoelectric conversion system with a circuit as shown in FIG. 3, a signal as shown in FIG. 4 is produced. Also, if a number of circuits as shown in FIG. 3 are arranged in a row, a one-dimensional image sensor can be provided.
As is apparent from the curve II in FIG. 2, a light receiving element which is provided by forming the optically transmissive SnO.sub.2 film 4 on the photoelectric film 3 by using the SnO.sub.2 target under the oxygen partial pressure 2.times.10.sup.-4 Torr and the gaseous pressure 3.5.times.10.sup.-4 Torr of the mixture of oxygen and argon has a considerably high electric current ratio for light and dark intensities.
If a 90% In.sub.2 O.sub.3 -10% SnO.sub.2 sintered body is used as the target, the sputtering process is carried out with a magnetron type sputtering device under an oxygen partial pressure (2.5.times.10.sup.-4 Torr) defined by the point B on the curve II in FIG. 5, the light receiving element so produced has a maximum electric current ratio for light and dark intensities. The electric current ratio is about 400.
However, if the optically transmissive electrode 4 is formed by sputtering under the oxygen partial pressure which is defined by the point B in FIG. 2 or 5, the specific resistance .rho. is increased as a result of which the resistance of the entire optically transmissive electrode 4 is increased.
Accordingly, an object of the invention is to provide a light receiving element in which, with the electric current ratio for light and dark intensities maintained large, the resistance of the entire optically transmissive electrode is made low.